The development of balloon-expandable coronary stents marked a significant advance in the treatment of coronary artery disease by providing an alternative to balloon angioplasty and bypass surgery. Stents are thin walled tubular-shaped devices which hold open a segment of a blood vessel. Stents are typically implanted in the “radially collapsed state” by a catheter which is inserted at an easily accessible location and then advanced through the vasculature to the deployment site. Once in position, the stent is deployed by inflation of a dilation balloon.
The stent must have the following properties:
1. adequate radial strength (hoop strength), capable of withstanding the structural loads exerted on the stent as it supports the walls of a vessel lumen;
2. longitudinal flexibility to allow it to be maneuvered through a tortuous vascular path;
3. sufficient ductility to provide the required flexibility during insertion and sizing at the deployment site;
4. conformity to the deployment site “geometry” that may not be round or straight and may be subject to flexure;
5. ability to be placed at or near branch points of vessels;
6. ability of the material to undergo sizing (compression and/or expansion) which requires substantial deformation of at least part of the stent's structure;
7. size retention, once expanded, throughout its service life and ability to keep withstanding the various forces including the cyclic loading induced by the beating heart;
8. biocompatibility;
9. ability to be sufficiently radiopaque or fluoroscopically visible under x-ray to allow accurate stent placement using real-time visualization enabling tracking the delivery catheter through the patient's vasculature and precise placement of the stent at the site of a lesion.
10. convenient and economic to manufacture with high production yield.
11. high reliability in use and adequate longevity.
Stents are typically made of stainless steel, cobalt alloys or nickel-titanium alloys, all of which, at the relatively thin wall thickness of about 50 to 150 micron, are sufficiently radiopaque to be visualized with x-ray based fluoroscopy procedures.
It is well known in the art that shaping and forming of metals (i.e., sheets, rods, tubes) using deformation steps in the processing results in the alignment of grains along certain preferred crystallographic orientations. Invariably all metal working processes introduce some directionality (i.e., texture) to the crystal structure and as a result, the anisotropic properties of single crystals of a metal are imparted to the polycrystalline aggregate in various degrees. Both elastic and plastic mechanical properties are dependent on crystallographic orientation.
The fundamental basis for understanding the deformation behavior of a crystalline solid and its dependence on crystal orientation is known as the Schmid law. According to this law, the plastic flow is carried by slip on a crystallographic slip system which is a certain combination of a crystallographic plane and a crystallographic direction in this plane. The deformation process in single crystals was studied extensively and the anisotropy of elastic modulus in single crystals has been summarized by Schmid and Boas (Schmid and Boas, Plasticity of Crystals, Hughes, London, 1950, p. 191). The degree of anisotropy varies considerably even from metal to metal of the same crystallographic system.
Depending on the nature of the crystallographic texture and the intended use of the material, anisotropy may or may not be a desirable feature from the practical viewpoint. It is often necessary to design a specific crystallographic texture for a particular purpose. A sharp texture in non-ferrous metal and alloys is generally considered to be undesirable because of the occurrence of earing and fracture during the fabrication process. In the fabricated metal products a completely random orientation is generally considered to be exceptional and highly desirable. This is also preferred for applications requiring a high degree of reliability such as coronary stents.
Stent tubes are typically made from drawn tubes or from rolled sheets. It is well documented that in f.c.c. (face-centered cubic) metals the deformation texture associated with tube forming is usually composed of [111] and [100] components (Hsun, Hu, Texture of Metals, Texture, vol. 1, p. 233-258, 1974). Even after annealing, the [111] and [100] duplex fiber texture is usually retained with some scattering. During placement of the metallic stents, the stent can undergo significant plastic deformation (i.e., from crimping and expansion procedures) in order to achieve the required diameter. The plastic deformation is known to create residual stresses. The dilation of the stents leads to a concentration of tensile and compressive residual stresses in different areas of the nodes of the stent. Since the stent is comprised of a network of small wire segments with a cross section of 50-150 microns in thickness and a nominal grain size of 25 to 30 microns, each stent wire essentially contains only a few grains in the thin cross-section. The accumulation of the residual stresses in such a thin section can therefore be influenced by the local grain crystallographic orientations and cause a weakening of the mechanical stability of the stent structure and reduce the reliability of the device during insertion and in service. The thin, multi-directional stent wires ideally require isotropic properties in all directions, a randomly oriented microstructure and a low texture intensity value to optimize the uniformity of deformation during emplacement and to maximize performance and reliability. Specifically, the reduction in the local texture variances has practical importance in minimizing the anisotropy arising from both the residual stresses during device implantation and the load stresses from cyclic heart beats while the device is in service. The specific benefits this invention provides therefore relate to the improvement in material performance (i.e., resistance to fatigue crack) and device longevity (i.e., stress corrosion cracking and dissection of the artery wall by a broken stent strut).
The patent literature on various features of stent designs is extensive. Generally stents are formed by e.g. laser cutting a tube to imprint a suitable pattern. Stent tubes are typically produced by drawing to form a thin metal tube of the appropriate dimensions, or by folding and welding a thin sheet. Other fabrication methods include direct forming e.g. using electroforming or sputtering.
Various patents disclose stents made from anisotropic and textured drawn tubes including thin walled and slotted structures produced from drawn tubes exhibiting a significant texture and anisotropic properties and include:
J. Palmaz in U.S. Pat. No. 4,739,762 (1988) discloses an expandable and deformable intraluminal vascular graft which is expanded within a blood vessel by an angioplasty balloon associated with a catheter to dilate and expand the lumen of a blood vessel. The graft may be a thin-walled tubular member having a plurality of slots disposed substantially parallel to the longitudinal axis of the tubular member.
R. Schatz in U.S. Pat. No. 5,195,984 (1993) discloses a plurality of expandable and deformable intraluminal vascular grafts which are expanded within a blood vessel by an angioplasty balloon associated with a catheter to dilate and expand the lumen of a blood vessel. The grafts may be thin-walled tubular members having a plurality of slots disposed substantially parallel to the longitudinal axis of the tubular members. Adjacent grafts are flexibly connected by a single connector member disposed substantially parallel to the longitudinal axis of the tubular members.
J. V. Donadio in U.S. Pat. No. 6,107,004 (2000) describes a manufacturing process for slotted tubes for use as catheters. The manufacturing process includes creating a pattern of slots or apertures in a flexible metallic tubular member by processes including electrostatic discharge machining (EDM), chemical milling, ablation and laser cutting. These slots or apertures may be cut completely or partially through the wall of the flexible metallic tubular member. These manufacturing processes may also include the additional step of encasing the flexible metallic member such that a fluid-tight seal is formed around the periphery of the tubular member.
Patents addressing forming stents from thin sheets exhibiting anisotropic properties and texture include:
J. Kula in U.S. Pat. No. 6,776,022 (2004) describes a flexible stent made from a metal sheet. The sheet is rolled in its central region to a specified wall thickness. Thereafter, the stent is photochemically etched to produce the desired cell pattern of the design of the stent. Then, the stent is folded and the metal is joined to give rise to a stent with multiple wall thickness. Typically, the ends of the stent comprise larger wall thicknesses whereas the wall thickness at its center remains smaller.
Various patents address cutting stents from anisotropic and textured tube-feedstock and include:
M. Reed in WO03072287A1 (2003) describes a methods for fabricating implantable medical devices having microstructures on tubular or cylindrical surfaces. A precursor tube fabricated from stainless steel, titanium, nitinol or tantalum, having an outer surface coated with a photoresist material, is treated to define an optical pattern on the photoresist material and the outer surface is etched electrochemically to form the desired microstructure. Thereafter, the photoresist material is removed and the device precursor is machined to form the implantable tubular medical device.
L. Lau in U.S. Pat. No. 5,421,955 (1995) describes an expandable stent for implantation in a body lumen, such as an artery, and a method for making it from a single length of tubing. The stent consists of a plurality of radially expandable cylindrical elements generally aligned on a common axis and interconnected by one or more interconnective elements. The individual radially expandable cylindrical elements consist of ribbon-like materials disposed in an undulating pattern. The stents are made by coating a length of tubing with an etchant-resistive material and then selectively removing portions of the coating to form a pattern for the stent on the tubing and to expose the portions of the tubing to be removed. This may be done by machine-controlled activation and relative positioning of a laser in conjunction with the coated tubing. After the patterning of the tubing, the stent is formed by removing exposed portions of the tubing by an etching process.
Patents addressing alternative methods for forming stents include:
Electroforming of stents is disclosed in R. A. Hines in U.S. Pat. No. 6,019,784 (2000). The process for making electroformed stents involves coating a mandrel with a resist, exposing portions of the resist to light to form a stent pattern, metal plating the mandrel and then dissolving the mandrel. As electroforming is a linear deposition process, the resulting stent would be expected to possess some degree of fiber texture and development of columnar grains in the radial direction.
A. Cohen in U.S. Pat. No. 6,790,377 (2004) describes an electroplating method to form a layer by i) contacting a substrate with a first article which includes a support and a conformable mask; ii) electroplating a first metal from a source of metal ions onto the substrate in a first pattern, the first pattern corresponding to the complement of the conformable mask pattern; and iii) removing the first article from the substrate. The method may further involve selectively or non-selectively depositing one or more additional materials to complete the formation of the layer, planarizing the deposited material after one or after each deposition step and/or forming layers adjacent to previously formed layers to build up a structure from a plurality of adhered layers. As electroplating is also a linear deposition process, the various adhered layers described in this patent would also be expected to possess some degree of fiber texture in the radial direction.
J. Palmaz in U.S. Pat. No. 6,820,676 (2004) describes implantable endoluminal devices which are fabricated from materials which provide a blood or body fluid and tissue contact surface and exhibits controlled heterogeneities in the material constitution. The stents are fabricated using vacuum deposition methods. As vacuum deposition is also a linear deposition process, the implant described in this patent would also be expected to possess some degree of fiber texture and development of columnar grains in the radial direction.
Until recently, conventional stents were produced in a straight tubular configuration. The use of such stents to treat vessels at or near a branch point is not without risk. Ideally, stents are tailor made to the specific location of placement, can be placed in locations where vessels branch, can be used to treat lesions in the “main” as well as the “side” branch of a vessel allowing for different stent shapes and sizes to be implanted.
Various patents address stents with a main tubular stent body having one or more side openings which may further comprise an extendable or second stent inserted through the side opening and at least partly in registry with the wall of the side opening.
G. Vardi in U.S. Pat. No. 6,599,316 (2003) describes stents for use in treating lesions at or near the bifurcation point in bifurcated cardiac, coronary, renal, peripheral vascular, gastrointestinal, pulmonary, urinary and neurovascular vessels and brain vessels. The invention discloses a stent apparatus with at least one side opening which may further comprise an extendable stent portion laterally extending from the side opening and at least partly in registry with the wall of the side opening. Devices constructed in accordance with the invention include a main expandable stent comprising at least one substantially circular side opening located between its proximal and distal end openings, where the side opening may further comprise an expandable portion extending radially outward from the edges of the side opening; and a branch stent comprising proximal and distal end openings which may further comprise a contacting portion at its proximal end, and which may optionally be constructed to form either a perpendicular branch or a non-perpendicular branch when inserted through a side opening of the main stent.
T. Chuter in U.S. Pat. No. 6,814,752 (2004) describes a system and method for treating and repairing complex anatomy characterized by a plurality of vessel portions oriented at various angles relative to each other. The system includes a graft device that is capable of being assembled in situ and has associated therewith a method that avoids the cessation of blood flow to vital organs. A delivery catheter system and various graft supporting, mating and anchoring structures are also included.
The novel stents proposed herein have essentially isotropic properties in all directions, exhibit little crystallographic texture and can be “tailor-made” to mimic the geometry of the blood vessel(s) at the deployment sites. The stents can be machined from metallic material feedstock which is ductile, corrosion resistant and has a quasi-isotropic, randomly oriented microstructure with no specific texture. Furthermore the stents can be designed to treat coronary artery disease at the point where blood vessels branch.